For the user of batteries it is of particular importance to know the state of charge and peak current loadability of the batteries. For example, for the ability of a battery to start a motor vehicle with an internal combustion engine the state of charge and the state of aging or the characteristic capacity loss of the battery are critical since the current intensity that can be taken off the battery and its power delivery are limited. Of particular importance is the determination of the state of charge or the starting capability of a battery in those cases in which, for example, intermittent motor operation occurs. This is important because, during times when the motor is not running, the on-board power system or the vehicle, with its various power consumers, continues to operate even though the generator is not producing any current. Monitoring the state of charge and starting capability of the battery in such cases assures that the energy content of the battery is sufficient to start the engine.
Various procedures are known for measuring the state of charge of batteries. In many cases integrating measurement devices are used (Ah counters), in which case the charging current, if necessary with involvement of a fixed charging factor, is considered. Since the useful capacity of a battery is strongly dependent on the magnitude of the discharge current and temperature, no satisfactory pronouncement concerning the useful capacity that can still be taken from the battery can be made with such procedures.
From DE-PS 2242510, for example, it is known in a process for measuring the state of charge that the charging current can be adjusted with a factor depending on the temperature and the state of charge of the battery itself.
DE-OS 4007883 describes a process in which the starting capacity of a battery is found by measuring battery voltage and battery temperature and comparison with a state of charge family of curves valid for the type of battery being tested.
DE-OS 19543874 discloses a calculation process for the discharge characteristic and residual capacity of a battery in which the current, voltage and temperature are measured, wherein the discharge characteristic is approximated by a mathematical function with a curved surface.
DE-PS 3901680 describes a process for monitoring the cold starting capability of a battery in which the battery is temporarily loaded with a resistance, the voltage drop at the resistance is measured and from this by comparison with anticipated values it is determined whether the cold starting capability of the battery is still sufficient. In this case, the starting process is used to load the battery.
Finally, DE-OS 4339568 discloses a process for determining the state of charge of a motor vehicle battery in which the battery current and rest voltage are measured and from these parameters the state of charge is determined, in which case the battery temperature is also considered. In this case, the charging currents are measured during different time intervals and compared with each other and from this the residual capacity is found.
By known procedures it is not directly possible to make a statement of sufficient accuracy regarding the state of charge of a battery. In addition, many of known procedures are costly and cannot always be used directly in the motor vehicle.